The invention relates to a process in for producing a layer system for gas sensors. German Offenlegungsschrift, or Laid Open Patent Application 29 04 069 discloses layer systems for gas sensors, in particular for lambda probes or polarographic probes, in which an interlayer of fully stabilised zirconium dioxide is formed on a partially stabilised solid electrolyte body before applying metallic or cermet electrodes. This layer sequence makes it possible to exploit the good mechanical strength and high thermal-shock resistance of partially stabilised solid electrolyte material, while at the same time, however, avoiding the disadvantages of these solid electrolyte materials in sensor manufacture. Irreversible transformation of metastable tetragonal ZrO.sub.2 into monoclinic ZrO.sub.2 produces microstructure damage, initially at the surface of the ceramic and then in the interior of the ceramic as well as a result of crack propagation. Monoclinic zirconium dioxide has, however, also the further disadvantage that it virtually does not conduct O.sup.-2 ions. The irreversible phase transformation tetragonal to monoclinic zirconium dioxide at the boundary surface between solid electrolyte body and electrodes therefore results in a further disadvantage, namely in a reduction of the electrolyte/electrode/test gas three-phase boundary and, consequently, in an impairment of the electrode function.
The phase transformation of the tetragonal ZrO.sub.2, which initially starts at the surface, is substantially suppressed, according to German Offenlegungsschrift, or Laid Open Patent Application 29 04 069, by applying an interlayer of fully stabilised ZrO.sub.2 between solid electrolyte body and electrode.
Furthermore, German Offenlegungsschrift, or Laid Open Patent Application 28 52 638 discloses the improvement of the load carrying capacity of gas sensors by using ceramic materials having different sintering activity for the layer system composed of solid electrolyte and cermet electrodes, in which connection the ceramic material for the supporting structure of the cermet electrode should be less sintering-active than the ceramic material of the solid electrolyte.